<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    :root {
      --top-color: orange;
      --bottom-color: yellow;
      --my-gradient: linear-gradient(var(--top-color), var(--bottom-color));
    }

    body {
      height: 100vh;
      background-image: var(--my-gradient);
    }
  </style>
</head>
<body>
  
</body>
</html>